The present invention provides a drive system for a variable diameter rotor blade system, and more particularly to a harmonic gear system to selectively extend and retract the variable diameter rotor blade system.
A tilt rotor or tilt wing aircraft typically employs a pair of rotor systems which are pivotable such that the rotors may assume a vertical or horizontal orientation. In a horizontal orientation (i.e., horizontal rotor plane), the aircraft is capable of hovering flight, while in a vertical orientation (i.e., vertical rotor plane), the aircraft is propelled in the same manner as conventional propeller-driven fixed-wing aircraft.
Variable Diameter Rotor (VDR) systems are known to provide distinct advantages. That is, when the plane of the rotor is oriented horizontally, the rotor diameter is enlarged for improved hovering efficiency and, when oriented vertically, the rotor diameter is reduced for improved propulsive efficiency.
One known example of a VDR blade assembly provides an outer blade segment configured to telescope over a torque tube member. A retraction/extension mechanism includes a planetary gear arrangement which is selectively drivable in either direction by a control shaft coaxially mounted within the rotor system drive shaft. The control shaft is selectively driven to extend and retract a reeling assembly attached to the outer blade segment. Controlling the extension and/or retraction of the outer blade segment relative to the torque tube thereby varies the rotor diameter.
The extension or retraction of the VDR system typically takes place in approximately 15-30 seconds. The remainder of the operational time there is no relative rotation between the planetary gear arrangement. An undesirable combination of high loads placed upon the non-rotating planetary gear arrangement may thereby cause fretting and pitting between the many intermeshed gears and bearings. Undesirable life reduction of the planetary gear arrangement may thereby result.
Accordingly, it is desirable to provide a VDR retraction/extension system which is light and compact while minimizing negative impact upon the VDR retraction/extension system when it is not being utilized.
The VDR retraction/extension system according to the present invention includes a reeling system driven by a harmonic gear system. The reeling system includes a strap drum rotatable about an axis of rotation. The harmonic gear system includes a ring gear, a flexible gear, and a wave generator. The harmonic gear system is mounted about the axis of rotation to selectively extend and retract each VDR blade assembly.
The ring gear is rigidly attached to the rotor hub assembly through a spline or the like, and includes a plurality of internal gear teeth within the ring gear inner diameter. The rotor hub assembly thus drives the attached ring gear.
The flex gear is attached to the strap drum and includes a plurality of external gear teeth about the flex gear outer diameter. The number of external flex gear teeth about the flex gear are less numerous than the number of internal gear teeth within the ring gear. The strap drum is thereby rotatably drivable by the attached flex gear.
The wave generator includes an elliptical disk enclosed within a bearing assembly. The wave generator is mounted within the bore of the flex gear to impart the shape of the elliptical disk to the flex gear.
In operation, a control shaft drives the wave generator at a higher or lower rotational speed relative to a main rotor drive shaft assembly thereby causing the wave generator to rotate relative to the rotor hub assembly. The wave generator may be driven in either direction depending upon the speed differential between the control shaft and the main rotor shaft. In turn, the wave generator causes the external flex gear teeth to sequentially engage the ring gear internal gear teeth as the wave generator rotates within the flex gear.
As the flex gear has fewer teeth than the ring gear, one revolution of wave generator causes relative motion between the flex gear and ring gear which equals the difference in teeth. As the flex gear is mounted to the strap drum, the differential rotation between the flex gear and ring gear causes the strap drum to rotate relative the rotor hub assembly. Rotation of the strap drum thereby causes the strap to be wound about or off of the drum and, consequently, effecting retraction/extension of each VDR blade assembly.
The present invention thereby provides a VDR drive system which is light and compact while minimizing any negative impact upon the VDR retraction/extension system when it is not being utilized.